The field of the present invention is valve drive systems capable of changing valve timing and valve lift.
Mechanisms have been known for varying valve timing and valve lift during engine operation. One such device is disclosed in Japanese Patent Laid Open No. Hei-3-130510. A camshaft support member is pivotally mounted to a head about a rocking axis. The camshafts mounted thereby engage rocker arms which in turn engage poppet valves. The rocker arms are pivotally mounted to the head such that movement of the camshaft support member about the rocking axis changes the point of engagement of the camshafts mounted thereto with the rocker arms. Associated with the upper portion of the camshaft support member is a gear segment. Mounted to the head cover is a worm gear and servomotor. The worm gear meshes with the gear segment on the camshaft support member to operatively position same.
Cam gears fixed to each camshaft move with the camshaft support member in the aforementioned device. Rotatably mounted to the head about the rocking axis is an idler gear engaging the cam gears. With rocking of the camshaft support member, the valve timing is changed. The cam gears are driven by the crankshaft with the cam gears rotating at half the speed of the crankshaft. Given the necessary driving ratio, given the need to place the center of the idle gear at the rocking axis and given the relative displacement of the cams from that axis, limitations are placed on the size and flexibility of the drive components.
The prior system, with the drive mechanism for the camshaft support member is mounted in the valve cover and with the limitations on the configuration for the cam drive have required compromises. The head cover cannot be loosely supported on the cylinder head. This allows the noise of the engine to be transmitted through the head cover and a larger head cover is required with a corresponding increase in height of the engine. The limitations on the camshaft drive system also can result in a larger overall structure and limitations on design freedom in terms of valve timing variations with valve lift control.